Apparently the conductivity is not improved that much over pure copper. The capacity of current carrying is increased. I guess the limit in copper is partially due to impurities and flaws in the crystilline structure (choke points). This does play a role in the improving conductivity of copper at lower temperatures. To get the maximal improvement in conductivity at cryogenic temperatures (~ 8 X at liquid nitrogen, and ~ 2 0X at liquid helium temperatures) the copper has to be very pure, which translates into very expensive. If this material bypasses the purity issues it may have a cost benefit if it follows a similar cryogenic conductivity trend.. Other than that, I'm not sure of the benefit for shrinking coils. With similar overall conuctivity, the current induced heating would be similar. Active cooling limitations would possibly still drive the capacity versus size of the magnets (at similar baseline temperatures).

It seams that I have heard that graphine nanotubes might have a conductivity as much as ~ 100 times over copper. If I am not confused, then the only advantage of this material with the copper would be possible cost/ quality control issues. ie: cheaper than very high purity copper, and actually available (if it is not just extrapolation from very small length tests without a clear path to up scaling production, just like all the other nanotube/ graphine research at this stage..

What is really needed is a superconductor (which of course has extremely high conductivity) which also has a very high ampacity, or at least ampicity comparable to good copper.

PS: I don't know the limits of copper. When transformer windings, electromagnet windings, etc. are chosen, given a certain quality of copper wire, are the engineers limited more by global heat isues or by local limitations where these defects/ contaminates/ choke points occur?

Dan,re: "With similar overall conuctivity, the current induced heating would be similar."Are you assuming the carbon-copper conductor with 100 times the current carrying capacity has the same resistance as the copper? Really, the heat conductance is not the same as current conductance, no? The point would be for the same size coil and coolant flow you would have 100 times the amp turns, or 20 times the performance of copper cooled with LN.Best regards,Mike

According to the quote from the article, the conductivity is the same as copper. Thus via Ohm's law reisistive heating at the same wire cross sectional area would be the same. Conductivity is the inverse of resistance. Assuming the copper and composite wires generate the same heat with the same current, then the improved current carrying capacity can only be due to the elimination of choke points- impurities/ inclusions in the copper wire that is the actual practical limiter on the copper wire ratings. Either that or the composite can tolerate much higher temperatures, thus cooling efficiency can be greater due to the increased delta T. This could perhaps allow for engineering advantages. Otherwise I am at a loss why this material could handle 100 times the resistive heating. I might add that DC resistance appears to be the same from my reading. I suppose induction heating may be different, they didn't say. If this material has different magnetic field / current properties, it may have advantages in AC current transmission, though I'm uncertain what the effects would be in transformers, motors etc. that are dependant on the magnetic effects.

Also, the quote may be misleading. Perhaps they meant the conductivity was 100 times better but it was confused in the article.

I'm reminded of reading someplace about someone saying that old used copper cable was different in some manner to new copper cable and was better for the task they had in mind, could using copper cable change it over time like this ?

This composite material has some differences from copper in current carrying capacity. The actual electrical conductance is similar to copper at similar temperatures, except at high temperatures the electrical conductance is mildly better. This may be similar to the difference between copper and tungsten? The article seems to stress the limit of copper for carrying current before the resistivity (one/ electrical conductance) starts increasing exponentially. This occurs at a few million Amps/ cm cross section in copper and a few hundred million amps in this composite. On the large scale, like household wiring or electromagnet wire, this is almost meaningless unless the current you are pushing is over ~ 10,000 Amps or more. But at the micro scale like traces in computer chips where diameters may be measured in a few dozen nanometers or less. This current carrying capacity before resistive run away is more critical as the diameter of the wires/ traces may be ~ 0.000001 X 0.000001 cm or ~ 10^-12 cm^2. At a capacity for ~2 million Amps / cm^2 in copper, the corresponding usable current carrying capacity in this tiny wire would be about 2 micro amps. This may be fine for running a few tiny transistors, but when you are talking about millions or even much more transisters on a integrated circuit, the current needed to operate all of these circuits may be be millions of times this. Example microprocessors may run at ~ 1 to 200 Watts, which at 5 volts would be about 0.2 to 40 Amps.

I don't know if this improved conductivity without run away resistivity increase is needed for individual traces, but for the power leads into major assemblies within a computer chip, I can envision this limit being reached. Avoiding it would have some design advantages in overall die size and the number of transistors per unit of area you could support.

The down side , which may be irrelevant, is how small this composite can be and retain its properties. Can it be one nanotube wide? Is a bundle of 100 nanotubes necessary for the cumulative properties?

In short, this material may have some design/ engineering advantages on the nano scale, but in the realm of electromagnets like that used in a Polywell or Tokamak, the issue is moot.

My limited understanding is that superconductors can carry up to ~ 10,000 Amps per cm^2 cross section of wire. I wasn't sure what copper's capacity was. It seams it is up to several million Amps per cm^2 (if you can keep it cool) WB6 operated at up to a few thousand amps through it's electromagnet wires. I don't know the cross section of the wire used, but I suspect it was in the region of ~ 0.02 to 0.05 cm^2. This would equate to a current capacity of about 20,000-50,000 Amps. This is ~ 10X that used in WB6, so I suspect the extra capacity would not be needed. The engineering of wire resistivity , cross sectional area, cooling requirements in terms of flow rate and thermal conductivity from the wires would not change. There would not be any advantage to this composite.

The exception may be at higher operating temperatures. Above ~ 80 degrees C this composite material does have some improved electrical conductivity relative to copper, so the necessary cooling may be reduced mildly. The opposite is the case below 80 degrees C (see the chart in the paper).

This all assumes that superconductors for the electromagnets are not an essential requirement. If they can design a practical combination of carbon (or boron?) nanotubes and /or 2 D graphene sheets rolled up in a copper matrix or other substance that has significant (like 10-100X) improved electrical conductance (less Ohmic heating) than copper at similar temperatures the attractiveness goes up considerably.

This engineering consideration of electromagnet windings, superconductors versus good or excellent non superconductor materials is a complex issue. A working superconductor is the obvious winner. But, this assumes the superconductor can operate and survive long enough in the severe B fields, X-ray, and possibly neutron environment. The shielding, low molecular weight (like water)shielding and cooling elements required for superconductors may eliminate much of the size considerations for a superconducting versus non superconducting magrid. The superconductor has an obvious advantage when considering the energy budget, but considerations of heat energy recovery through a water or CO2 steam cycle mitigates this somewhat.

I don't think I'm getting this. For whatever strength magnetic field we aim for, we need a certain amount of amp-turns. If this composite wire can carry 100x more current (amps), then we should be able to get by with a lot less copper. So space for cooling can increase, EVEN while overall size of rings can decrease ...

Tyler Jordan wrote:I don't think I'm getting this. For whatever strength magnetic field we aim for, we need a certain amount of amp-turns. If this composite wire can carry 100x more current (amps), then we should be able to get by with a lot less copper. So space for cooling can increase, EVEN while overall size of rings can decrease ...

Basically, my understanding is that this copper -buckytube composit has the same conducxtivity as copper. There is no difference in resistive heating or needs for cooling. The difference only manifests at stupendous current density in the wires and this does not occur in typical applications. It only applies when unrealistic currents pass through a mundane wire, or when smaller currents pass through extreamly tiny wire. At some small scale, such as with nanowires in electric circuits this threashold may be reached .

Current / wire diameter (or cross section) is the formula. Increasing the current to ridiculous levels or shrinking the wire to tiny sizes can reach this threshold where the conductivity of copper wire and this composite change. Outside of these extreams the materials are essentially the same.

Superconductors are best, if they can be made to work with the heat loads in a fusion reactor, there is a lot of heat from x-rays and possibly neutrons being deposited in a magnet coil. You have to keep the superconductor cold and carry away the applied heat. The cooling nessisary to maintain the required delta T may be greater for superconductors compared to resistive copper wire windings. The cooling plant may cost much more. But the savings in power lost in conventional conductors can add up to millions of watts, or even billions of watts in a large Tokamak. Quenching is a problem which probably be addressed, at uncertain costs. Failure in a relatively small Polywell is damaging to the coils, but no a danger to the plant or neighborhood. The case for large superconducting Tokamaks is a more challenging and dangerous situation. The complexity of the needed engineering and associated costs have to be compared to the superconducting magnet power and to an uncertain degree the cooling requirements. Of course superconductors that do not quench up to several hundred degrees C tremendously reduces the challenges. If only...

An example of exceeding the current capacity of wire where this composit would then be useful:1 cm high voltage transmission wire. At 200,000 volts and 100,000 amps, the wire has no problem- it is well below the threshold where the copper conductivity starts to change rapidly- ~ 3 million amps. And, this wire is carrying ~ 10 Gagawatts of power. No need for the composite in any realistic power requirement. Because the denominator in a nanowire is so small, the equation results change. A nano wire at 10 nanometers diameter can only carry about 3 million A * (0.0000001cm)^2 before the threshold is reached. That would be about 3 * 10^-8 amps or about 3 nanoamps. Using this composite that could be increased to ~ 200 nanoamps. In the world of nanocircuity this could be extreamly usefull. In normal scales we do not use such energy density.

Keep in mind that both conductors are heating up in proportion to the current to the same extent below this threshold. The change only applies at tremendous energy densities where current exceeds ~ 3 million Amps per square cm of wire cross section.

Naw ... I'm just an idiot I think. I'll re-read this a few times and do more homework. Your examples sort-of make sense to me ... but I keep thinking powerful pulsed magnet and amp-turns, not high voltage wire. I get that heat output is going to be greater at more amps in either case, so cooling is critical. But, alas, I'm not getting the rest of it yet.

D Tibbets wrote:Current / wire diameter (or cross section) is the formula. Increasing the current to ridiculous levels or shrinking the wire to tiny sizes can reach this threshold where the conductivity of copper wire and this composite change. Outside of these extreams the materials are essentially the same.

What is this the formula for?

...wait, maybe I get it now - Is the real utility of the carbon nanotubes only (or mostly) to increase the physical strength of the material? And hence it can carry more current before breaking? Maybe that's what I've been missing.

Resistance of materials generally goes up when you heat it. This limits the capacity.

This composite doesn't resist as much as higher temperatures, and in fact is a hundred times better.

So, the wire in your walls won't care, it doesn't carry enough current to get hot enough to matter.

The teeny tiny wires in a microchip can hit the currents expected, so it might be useful there, since it'd be one less thing to fail due to heat, potentially letting you run the chip hotter, which would be good because you'd be able to worry less about overheating.

For a polywell, it'd also be good, because you have a similar size limitation with large currents. With the expectation of running hotter, you'd be able to use a smaller wire that lets you put more turns in the same space. How does the stuff compare with the liquid nitrogen level superconductors we'd be expecting to use instead?

I don't think the size of the wire matters for generating a high magnetic field at least with regular copper. It is amp-turns that gives us a high magnetic field - so high amperage and low turns equals lower amperage and high turns. With normal copper the thicker the wire the more amps you can run through it, so for long-term durability/performance, going with the thickest possible conductor -- something that perhaps looks like a bitter magnet (using plates instead of wire) but isn't a bitter magnet as we're not trying to compress the magnetic field in the center. hence we can run enormous current, not have to worry about insulation problems, space between wires is eliminated (so reducing overall size), cooling is probably easier too, and above all the strength of the conductor is greatly improved reducing deformation.

if we are cooling such a magnet with liquid Nitrogen ... our resistance is low, so cost of running the magnet is much improved (although we would have to factor in cooling cost). We don't have to worry about quenching at all, so we can push magnets right up to whatever we level we desire.

given that scenario, I don't see the advantage of the copper composite unless it can handle a lot more current under such a scenario? - my take-away so far is that it cannot, and, at least for a test machine, I think pure copper looks better than superconductors. As we can push copper magnets under pulsed conditions to super high magnetic fields, if desirable, and it might be.

one thing - the composite has a lower density. if it's thermal conductivity is still relatively the same as pure copper (a big if?), then this should translate as being easier to cool.

.... then again, I may be totally wrong about any number of things here